The use of caesium fluoroaluminates as fluxes for soldering aluminium materials is known. The low melting point and hence the ability to solder even magnesium-containing aluminium alloys opens up many applications and developments. In the automotive industry, for example, the trend in recent years has been towards ever smaller radiators, which operate at ever greater pressures. This has led to increased strength requirements for the radiators, and this has been reflected in a higher magnesium content in the aluminium alloys. This development has also given rise to greater requirements for fluxes. In order to meet these requirements, it must be possible to tailor a flux specifically to the individual application. A critical requirement here is the ability to adjust the melting point of a flux across a broad range and above all also to allow a low melting point or melting onset below 440° C. The flux should be resistant, to oxidation for example, and be able to be processed into pastes and soldering rods.
The prior art offers fluxes made from crystalline complexes of caesium/aluminium/fluorine (Cs/Al/F), which are known as caesium fluoroaluminates. A disadvantage of these crystalline substances for use as fluxes is that they are defined compounds with narrow melting ranges or even precise melting points. Moreover, additional substances such as additives for example, with which the basicity or acidity can be adjusted and which have a positive influence on the soldering characteristics, cannot be bound to the crystalline complex.
U.S. Pat. No. 4,689,092 describes a flux consisting of a caesium fluoroaluminate complex which with an appropriate ratio of caesium fluoride (CsF) to aluminium fluoride (AlF3) begins to melt at 440° C. The material exhibits strong crystallinity in the X-ray diffractogram and is not hygroscopic. The material is technically simple to produce by a hydrothermal process from caesium fluoride and aluminium fluoride. However, melting points below 440° C. cannot be achieved; melting points can only be established in the range from 440 to 460° C. Furthermore, the flux does not exhibit good properties for the soldering of materials whose magnesium content is greater than 1%. Moreover, due to its ready oxidisability and its rapid degradation, this flux is not suitable for flame soldering (U.S. Pat. No. 5,171,377).
U.S. Pat. No. 4,923,530 describes the production of an oxygen-containing suspension of caesium fluoroaluminate, which has good stability and a low melting point of 414° C. However, hydrofluoric acid at temperatures of up to 90° C. is used in its production. The handling of the highly toxic hydrofluoric acid under these conditions calls for special requirements for materials and occupational safety. The use of caesium carbonate (Cs2CO3) in the reaction generates CO2, which forms aerosols containing hydrofluoric acid and fluoride. Furthermore, the aim of this process is to produce a suspension. However, this is unsuitable for the production of soldering rods or anhydrous pastes. A dry powder is needed for that purpose.
U.S. Pat. No. 5,171,377 describes the production of a flux from caesium fluoride, aluminium fluoride and crystalline aluminium hydroxide or aluminium oxide. The complex that is formed permits a broad melting range and is suitable for soldering magnesium-containing aluminium alloys. However, a melting onset below 440° C. cannot be achieved. The use of aluminium hydroxide or aluminium oxide as additives means that no acidity can be established in the flux. The acidity of the flux counteracts the oxidation of the flux.
EP-A-0785045 describes the production of a flux consisting of a caesium fluoroaluminate reaction product with varying ratios of CsF to AlF3. Depending on the ratio of CsF to AlF3, melting points of less than 440° C. down to 427° C. can be achieved with this flux. However, the flux has clearly defined melting points, which shows that it is a crystalline substance.
Thus the fluxes known from the prior art do not correspond to the requirements described in the introduction. In particular, the prior art describes no amorphous caesium fluoroaluminates as fluxes.